preheater for combustion air, and a power plant

ABSTRACT

A preheating device for combustion air in a boiler. First heat exchanger structures fitted in a flue gas duct heat primary air. A first air supply area in the wall of the flue gas duct supplies air to be heated to the first heat exchanger structures. Second heat exchanger structures fitted in a flue gas duct heat secondary air. A second air supply area in the wall of the flue gas duct supplies air to be heated to the second heat exchanger structures. The first air supply area is opposite the second air supply area. Also a power plant.

FIELD OF THE INVENTION

The invention relates to a device for preheating combustion air by flue gas in the boiler of a power plant. Furthermore, the invention relates to a power plant comprising a device for preheating combustion air to be supplied into a boiler by means of flue gas.

BACKGROUND OF THE INVENTION

For preheating combustion air for a solid fuel boiler, flue gas air preheaters (LUVOs) are typically used, in which the heating medium, i.e. flue gas, flows outside heat exchanger pipes, and the medium to be heated, i.e. air, flows inside the heat exchanger pipes. The heat exchanger pipes are placed horizontally in the flue gas duct, and the heat exchanger units on different levels are connected to each other by air ducts outside the flue gas duct. There are also configurations, in which the flue gas flows inside the pipes and the pipes are vertical.

A typical combustion air preheating device 1 is shown in FIG. 1. This kind of a preheating device 1 comprises horizontal heat exchanger structures 3 placed in a flue gas duct 2 for heating primary air P, and heat exchanger structures 4 for heating secondary air S. Separate air supply areas 5, 6 for both air circulations are provided on the wall of the flue gas duct 2, for supplying air to be heated into the heat exchanger structures 3, 4. In this configuration, the coupling of air ducts is made as advantageous as possible for the layout of the plant. Primary air P and secondary air S are supplied into the preheater 1 from the same side. This configuration causes a strong distortion in the temperature of the flue gases F, because all the cold air is supplied into the preheater 1 from one side, causing significantly stronger cooling of the flue gases on the air inlet side than on the opposite side. The strong cooling of the flue gases F on the air inlet side, in turn, causes a relatively low material temperature at the air inlet end of the pipe of the heat exchanger structure 3, 4 of the preheater 1, in spite of the relatively high average temperature of the flue gases. Thus, the dew point of flue gases is easily achieved at the surface of the heat exchanger structure 3, 4. The dew point, in turn, will cause strong corrosion in the cold heat exchanger structure 3, 4 and erosion in a short time, particularly with difficult fuels.

BRIEF SUMMARY OF THE INVENTION

Now, a solution has been found which enables a more uniform temperature distribution in the terminal part of the flue gas duct.

To achieve this aim, the combustion air preheating device according to the invention is primarily characterized in what will be presented in the independent claim 1. The power plant according to the invention is, in turn, primarily characterized in what will be presented in the independent claim 8. The other, dependent claims will present some preferred embodiments of the invention.

The basic idea of the invention is to form a preheating device for the combustion air of the boiler by providing the flue gas duct with first heat exchanger structures for heating primary air and second heat exchanger structures for heating secondary air. The inlet of the first heat exchanger structure and the inlet of the second heat exchanger structure are placed substantially on the same level, the level being substantially perpendicular to the central line of the flue gas duct.

In an embodiment, the first heat exchanger structures and the second heat exchanger structures are staggered.

In an embodiment, the first heat exchanger structures and the second heat exchanger structures extend at their air supply areas to the central area of the flue gas duct provided with air flow deflecting structures for changing the direction of air flows of the heat exchangers. The deflecting structure may be, for example, a chamber or a bent pipe.

In an embodiment, the first heat exchanger structures and the second heat exchanger structures are formed of pipes by bending so that the straight portions of the pipes extend both parallel to the central line of the flue gas duct and perpendicular to the central line of the flue gas duct.

A power plant according to the basic idea of the invention comprises at least a boiler and a flue gas duct, into which the flue gases exiting the boiler are led. Furthermore, the plant comprises the above-described preheating device for heating the combustion air to be supplied into the boiler. In an advantageous embodiment, the flue gas duct is vertical, and the flue gases from the boiler are introduced from the upper part of the flue gas duct and discharged from the lower part.

In an embodiment, the first air supply area and the second air supply area are placed substantially at the same height.

In an embodiment, the first air supply area and the second air supply area are the lowermost parts of the preheater which are placed in the flue gas duct.

The different embodiments of the above-described configuration, taken separately and in various combinations, provide various advantages. A single embodiment may comprise one or more of the following advantages depending on its implementation:

-   -   the distortion of the flue gases can be leveled out;     -   the temperature difference between the flue gases and the         combustion air can be made as great as possible even in the         lowermost pipe rows, in both the front and rear parts of the         preheating device;     -   the material temperature of the pipe of the preheating device         can be raised at the location of the air inlet flow;     -   the duct arrangements of the plant are simpler than in         conventional configurations;     -   in the area of the bent pipe element, problems of vibration of         the preheating device are possibly reduced, because in one         embodiment, the pipes are tied together in the vertical portion;

in one embodiment, the problems of vibration of the preheating device are reduced, because the bundle comprises a primary air pipe and a secondary air pipe adjacent to each other; vibrations possibly caused by blowers on the primary and secondary air side have different frequencies, wherein they do not amplify each other.

One advantageous embodiment has the advantage that the cold bundles of both primary air and secondary air are placed as a single lowermost bundle. Thus, when replacing a damaged bundle is necessary, only a single bundle has to be replaced. Normally, when e.g. a corrosion problem occurs, it is usually necessary to replace both the lowermost primary and secondary air bundles according to a given schedule. It is particularly difficult to replace the lowermost bundle but one.

In an application according to one embodiment of the invention, an air deflecting chamber in the centre of the primary and secondary air bundles increases advantageously the flow rate of the flue gases at the end part of the flow gas duct, improving the heat transfer on the flue gas side. This contributes to raising the material temperature of the pipes of the preheating device in the most critical cold part and thereby reduces the risk of erosion of the pipes by the effect of the acid dew point.

In an advantageous embodiment, the heat transfer surface of the preheating device consists, in its entirety, of a pipe element extending from the bottom to the top. The elements are made, for example, on a bending line for superheater pipes. Of the adjacent elements, one is for primary air and the other for secondary air. The sizes of the pipes may be equal or different, depending on the reciprocal proportions of primary and secondary air. The pipe spacing may be, for example, typical 75×75 mm, or the spacing can be selected to provide a desired flue gas rate. In addition to the advantages presented above, the structure provides, among other things, the following advantages to conventional configurations:

-   -   the connecting channels between bundles on the air side are         totally eliminated, providing significant cost savings in the         manufacture of ducts;     -   the elimination of the connecting channels saves space in the         boiler room;     -   the installation of the preheating device becomes faster,         because there is no need to install connecting air channels;     -   the preheater device is divided into installation units in the         width direction of the preheater device;     -   the pressure loss on the air side is smaller than in a         conventional preheater device, because the inflows and outflows         of pipes between bundles are eliminated;     -   the single resistance coefficient on the 90° curve is lower than         in inflow or outflow of the pipe. This saves the internal         consumption of the boiler.

DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail with reference to the appended principle drawings, in which

FIG. 1 shows a combustion air preheater of prior art in a principle view;

FIG. 2 shows a power plant in a principle view;

FIG. 3 shows a first embodiment of a preheater;

FIG. 4 shows a second embodiment of a preheater;

FIG. 5 shows a third embodiment of a preheater;

FIG. 6 shows a fourth embodiment of a preheater;

FIG. 7 shows a detail of the preheater of FIG. 8;

FIG. 8 shows a fifth embodiment of a preheater;

FIG. 9 shows a cross-section along line A-A in FIG. 8;

FIG. 10 shows a detail of the preheater of FIG. 9;

FIG. 11 shows a detail of the preheater of FIG. 12;

FIG. 12 shows a sixth embodiment of a preheater;

FIG. 13 shows a cross-section along line B-B in FIG. 12;

FIG. 14 shows a seventh embodiment of a preheater;

FIG. 15 shows an application of a preheater.

For the sake of clarity, the drawings only show the details necessary for understanding the invention. The structures and details that are not necessary for understanding the invention but are obvious for anyone skilled in the art have been omitted from the figures in order to emphasize the characteristics of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows, in a principle view, a power plant with a combustion air preheater 1 in a flue gas duct 2. In the example, the power plant comprises a circulating fluidized bed boiler 7, but the boiler may also be of another type. As it can be seen in the figure, the flue gas duct 2 is between the boiler 7 and the stack 8. Furthermore, the flue gas duct may comprise superheaters and filters which are not shown in the figure.

In the example of FIG. 2, the lower part of the preheater 1 comprises air inlet areas 5, 6, through which the primary air P and secondary air S to be heated are supplied to the heat exchanger structures, that is, the heat transfer surfaces, of the preheater. The air supply areas 5, 6 for primary air P and secondary air S are placed on opposite sides of the flue gas duct 2, substantially on the same level.

The heat exchanger structures terminating in the air supply areas 5, 6 are, in the flowing direction of the flue gases F, the last ones of the heat exchanger structures of the preheater 1 placed in the flue gas duct 2. In the examples, the flue gas duct 2 is vertical, and the flue gases F flow downwards from an upper level so that the air supply areas 5, 6 are placed lowermost. In the beginning (that is, in the example, on the upper level), the flue gases F are hotter than in the end (in the example, on the bottom), so that the combustion air P, S can be made hotter when it is heated last before the outlet at the upper part of the preheater 1.

A basic idea of the configuration is that the air supply areas 5, 6 are perpendicularly facing each other on the opposite walls of the flue gas duct 2. In the horizontal flue gas duct 2, this means that the air supply areas 5, 6 are opposite each other, substantially at the same height. If the flue gas duct 2 is in another position, the air supply areas 5, 6 are in the same zone, which zone is perpendicular to the central line 2× of the flue gas duct 2.

The primary air P and secondary air S heated with the preheater 1 are guided with suitable channel structures from the upper part of the preheater into the boiler 7. In the example, the primary air P is supplied to the inlet of fluidized air and the secondary air S is supplied to the air supply level on the wall of the boiler 7. In the preheater 1, the combustion air P, S is heated to a temperature of about 150 to 250° C.

The heat exchangers of the preheater 1 consist of one or more units. In this context, a unit refers to an aggregate, in which the ends of the pipes extend from one wall of the flue gas duct 2 to another wall of the flue gas duct. Preferably, a unit in a vertical flue gas duct 2 comprises 20 to 30 pipes on top of each other and about a hundred pipes adjacent to each other.

Furthermore, one unit may consist of one or more subunits connected to each other. Preferably, a unit consists of two or more subunits. The building up of a preheater to be assembled of several subunits is often easier than the building up of a preheater of a single large unit.

FIGS. 3, 4, 5, 8, and 12 show applications, in which the preheater 1 for combustion air consists of several units. The pipes of the heat exchanger structures which are primarily effective on the heat transfer are placed transversely to the flowing direction of the flue gases F in the flue gas duct 2. In the configurations shown in FIGS. 3, 4 and 5, the lowermost units 9 are for both primary and secondary air P, S. The lowermost units 9, to which the air to be heated with the preheater 1 is supplied from the air supply area 5, 6, are called cold units. In the configurations according to FIGS. 3, 4 and 5, the uppermost units 10 have a structure similar to the units of a conventional preheater; that is, solely either primary air P or secondary air S is supplied into a single unit.

The pipes of the same air circulation can be adjacent to each other, wherein the structure is similar to that of FIG. 3. In the configuration of FIG. 3, the pipes of the primary air circulation P and the pipes of the secondary air circulation S are on opposite sides of the flue gas duct 2. The configuration can also be implemented in such a way that every other pipe element is for primary air P and every other one is for secondary air S, wherein the structure is similar to that of FIG. 4.

In an application according to an embodiment of the invention shown in FIG. 5, an air deflecting chamber 11 is provided as a deflecting structure for the air flow in the centre of the primary and secondary air structures in the lowermost unit 9. The air deflecting chamber 11 comprises separate facilities for both the primary and the secondary air circulation. Preferably, the air deflecting chamber 11 that reduces the cross-sectional area of the flue gas duct 2 increases the flow rate of flue gases F at the terminal part of the flue gas duct, improving the heat transfer on the flue gas side. Furthermore, the embodiment of FIG. 5 is advantageous for the replaceability of the units, because the cold heat exchanger structures of both air circulations are placed lowermost of the units 9, 10 of the preheater. In the configuration of FIG. 5, the lowermost unit 9 consists of pipe portions 12 and deflecting chambers 11. The primary air circulation P comprises two sections 12 consisting of pipes and the space of a deflecting chamber 11 connecting the same. In a corresponding manner, the secondary air circulation S consists of two portions 12 consisting of pipes and the space of a deflecting chamber 11 connecting them. As can be seen in the example, the inlets 5, 6 and outlets 13 of the air circulation of the lowermost unit 9 are on different levels. The outlet 13 is before the inlet 5, 6 in the flowing direction of the flue gases F. In this vertical flue gas duct 2, in which the flue gases F flow from an upper level downwards, the outlet 13 is higher than the inlet 5, 6.

FIG. 6, in turn, shows an application, in which the preheater 1 consists of a single unit 14. In the presented embodiment, the heat exchanger structures of the preheater 1 are implemented with continuous pipes extending from the cold end to the hot end. In this advantageous embodiment, the heat transfer surface of the preheating device 1 consists, in its entirety, of a pipe element extending from the bottom to the top. Also in this configuration, the pipes of the heat exchanger structures which are primarily effective on the heat transfer are placed transversely to the flowing direction of the flue gases F in the flue gas duct 2. In an advantageous embodiment, one of the adjacent elements is for primary air and the other for secondary air. The sizes of the pipes may be equal or different, depending on the reciprocal proportions of primary air P and secondary air S. The diameters of the pipes can be selected according to the need, to be different for the primary and secondary air side P, S. The pipe spacing may be, for example, normal 75×75 mm, or the spacing can be selected to provide a desired flue gas rate. In an embodiment, the primary and secondary air circulations P, S are interleaved so that in a recurrent series, there are two pipes of the primary air circulation and two pipes of the secondary air circulation adjacent to them. In another embodiment, in turn, in a recurrent series, there are two pipes of the primary or secondary air circulation, and one pipe of the secondary or primary air circulation in between.

FIGS. 7 to 10 show an embodiment, in which the upper part 15 of the heat exchanger structure of the preheater is implemented with continuous pipes. In the cold end of the heat exchanger structure, in turn, an air deflecting chamber 11 is used as a deflecting structure for the air flow in the centre of the primary and secondary air units. FIG. 7 shows a partial enlargement of a detail in FIG. 8, showing how the pipes of the unit have two deflections of 90°.

FIG. 9 shows a cross-section of location A-A in FIG. 8. FIG. 10 shows a partial enlargement of a detail in FIG. 9. From FIGS. 9 and 10, it can be seen how the pipes of the unit are placed on different levels t1-t6 of the preheater 1. In the example of FIG. 10, the pipes of the primary air circulation P are shaded, and the pipes of the secondary air circulation S are unshaded. In this example, broken lines are used to indicate the correspondence of some pipes on different levels t4, t5.

FIGS. 11 to 13 show another embodiment, in which the upper part 16 of the heat exchanger structure of the preheater 1 is implemented with continuous pipes. Also in this example, in the cold end of the heat exchanger structure, an air deflecting chamber 11 is used as a deflecting structure for the air flow in the centre of the primary and secondary air units. FIG. 11 shows the two outermost pipes 16 a of the upper part 16 of the primary air circulation P. It can been seen in the figure that the heat exchanger structures have been made of pipes by bending so that the straight portions of the pipes are placed both parallel to the central line 2× of the flue gas duct 2 and perpendicular to the central line of the flue gas duct.

Preferably, the pipe sections parallel to the central line 2× of the flue gas duct 2 are connected to each other, for example by binding. Thus, possible displacement and vibration of the pipes is reduced, and thereby also problems of vibration of the preheating device 1 are reduced.

FIG. 13 shows a cross-section of location B-B in FIG. 12. In the figure, it can be seen how the pipes of the unit are placed on different levels t1-t6 of the preheater. A substantial difference between the examples shown in FIGS. 7 to 10 and in FIGS. 11 to 13 is the density at which the pipes are connected to the upper part of the deflecting chamber. In the configuration of FIGS. 11 to 13, the horizontal distance between adjacent pipes is substantially equal to the horizontal distance between adjacent pipes in the upper part of the preheater. In other words, the horizontal distance between adjacent pipes is substantially the same on all levels t1 to t6.

In the configuration shown in FIGS. 7 to 10, the horizontal distance between pipes of the secondary air circulation S in the upper part t5 of the deflecting chamber 11 is substantially equal to the horizontal distance between pipes of the secondary air circulation in the upper part t1-t4 of the preheater 1. Also, the horizontal distance between pipes of the primary air circulation P in the upper part t5 of the deflecting chamber 11 is substantially equal to the horizontal distance between pipes of the primary air circulation in the upper part t1-t4 of the preheater 1. In the lower part t6 of the deflecting chamber 11, the density of the pipes is the same as in the upper part t1-t4 of the preheater 1. On the other side of the deflecting chamber 11, all the pipes of the lower part t6 are pipes of the primary air circulation P, and those on the other side are pipes of the secondary air circulation S.

FIG. 14 also shows an embodiment, in which the upper part 15 of the heat exchanger structure of the preheater 1 is implemented with continuous pipes. In this embodiment, a supply chamber 11′ is used for supplying for the primary and secondary air units at the cold end 9 of the heat exchanger structure. The supply chamber 11′ is placed in the centre of the lower part of the flue gas duct 2 where the air to be heated is introduced from the outside of the flue gas duct, preferably from the ends of the supply chamber 11′. The supply chamber 11′ comprises both the first air supply area 5 and the second air supply air 6 for supplying the heat exchanger structures with air to be heated. The primary air P and the secondary air S are supplied from the supply chamber 11′ in different directions, and as it can be seen in the figure, the air supply direction in the first air supply area 5 is at an angle of 180° to the air supply direction of the second air supply area 6.

Furthermore, for cleaning and maintenance of the preheater 1, the device comprises other structures than the above-described units of the preheater. The figures show sooting means 17, service hatches 18 as well as an ash removal opening 19. Furthermore, the device may also comprise other structures and parts which are not shown in the figures.

In the above-presented examples, the flue gas duct 2 is vertical, and the flue gases F flow downwards from an upper level, so that the air supply areas 5, 6 are placed lowermost. The flue gas duct may also be implemented in another way. For example, in FIG. 15, the flue gas duct 2 is vertical so that the flue gases F flow from the bottom upwards, wherein the air supply areas 5, 6 are placed uppermost. The flue gas duct may also be, for example, totally or partly horizontal, in which case the flue gases flow in the horizontal direction. At the beginning, the flue gases are hotter than at the end, and the combustion air can be made hotter when it is heated last before the outlet at the initial end of the flue gas duct.

By combining, in various ways, the modes and structures disclosed in connection with the different embodiments of the invention presented above, it is possible to produce various embodiments of the invention in accordance with the spirit of the invention. Therefore, the above-presented examples must not be interpreted as restrictive to the invention, but the embodiments of the invention may be freely varied within the scope of the inventive features presented in the claims hereinbelow. 

1-13. (canceled)
 14. A preheating device for combustion air in a boiler, the device comprising: first heat exchanger structures fitted in a flue gas duct for heating primary air, the first heat exchanger structures comprising a set of heat exchanger pipes forming a primary air circulation for primary air flow; second heat exchanger structures fitted in the flue gas duct for heating secondary air, the second heat exchanger structures comprising a set of heat exchanger pipes forming a secondary air circulation for secondary air flow; an inlet of the first heat exchanger structure for supplying primary air to be heated to the primary air circulation; and an inlet of the second heat exchanger structure for supplying secondary air to be heated to the secondary air circulation; wherein the inlet of the first heat exchanger structure and the inlet of the second heat exchanger structure are placed substantially on a same level, the same level being substantially perpendicular to a central line of the flue gas duct.
 15. The preheating device according to claim 14, further comprising: a first air supply area in a wall of the flue gas duct for supplying air to be heated to the first heat exchanger structures, the first air supply area comprising the inlet of the first heat exchanger structure; a second air supply area in the wall of the flue gas duct for supplying air to be heated to the second heat exchanger structures, the second air supply area comprising the inlet of the second heat exchanger structure; and wherein the first air supply area is opposite the second air supply area.
 16. The preheating device according to claim 15, further comprising: deflecting structures for the air flow for changing the direction of the air flows of the heat exchangers, wherein the deflecting structures are arranged in a central area of the flue gas duct, and wherein the first heat exchanger structures and the second heat exchanger structures extend at the first air supply area and the second air supply area to the central area of the flue gas duct.
 17. The preheating device according to claim 14, wherein the first heat exchanger structures and the second heat exchanger structures are staggered.
 18. The preheating device according to claim 14, wherein the heat exchanger pipes are bent such that straight portions of the heat exchanger pipes are placed both parallel to the central line of the flue gas duct and perpendicular to the central line of the flue gas duct.
 19. The preheating device according to claim 14, wherein the flue gas duct is vertical, whereby the inlet of the first heat exchanger structure and the inlet of the second heat exchanger structure are located at a same height.
 20. A power plant, comprising: a boiler; a flue gas duct to which flue gases exiting the boiler are led; a preheating device for heating combustion air to be supplied into the boiler, the preheating device comprising: first heat exchanger structures fitted in the flue gas duct for heating primary air, wherein the first heat exchanger structures comprise a set of heat exchanger pipes forming a primary air circulation for primary air flow, second heat exchanger structures fitted in the flue gas duct for heating secondary air, wherein the second heat exchanger structures comprise a set of heat exchanger pipes forming a secondary air circulation for secondary air flow, an inlet of the first heat exchanger structure, for supplying primary air to be heated to the primary air circulation, and an inlet of the second heat exchanger structure, for supplying secondary air to be heated to the secondary air circulation, wherein the inlet of the first heat exchanger structure and the inlet of the second heat exchanger structure are placed substantially on a same level, the same level being substantially perpendicular to a central line of the flue gas duct.
 21. The power plant according to claim 20, wherein the preheating device of the power plant further comprises: a first air supply area in a wall of the flue gas duct for supplying air to be heated to the first heat exchanger structures, the first air supply area comprising the inlet of the first heat exchanger structure; a second air supply area in the wall of the flue gas duct for supplying air to be heated to the second heat exchanger structures, the second air supply area comprising the inlet of the second heat exchanger structure; and wherein the first air supply area is opposite the second air supply area.
 22. The power plant according to claim 20, wherein the flue gas duct is vertical, whereby the inlet of the first heat exchanger structure and the inlet of the second heat exchanger structure are located at a same height.
 23. The power plant according to claim 21, wherein the first air supply area and the second air supply area are lowermost parts of the preheater placed in the flue gas duct.
 24. The power plant according to claim 21, wherein the first air supply area and the second air supply area are uppermost parts of the preheater placed in the flue gas duct.
 25. The power plant according to claim 22, wherein the flue gases from the boiler are introduced into the flue gas duct from an upper part of the flue gas duct and removed from a lower part.
 26. The power plant according to claim 22, wherein the flue gases from the boiler are introduced into the flue gas duct from a lower part of the flue gas duct and removed from an upper part. 